Pressure sensor having a multiple wheatstone bridge configuration of sense elements

ABSTRACT

A pressure sensor includes a diaphragm suspended across a cavity in a substrate. A first group of piezoresistors is provided in the diaphragm proximate a first outer edge of the diaphragm, the piezoresistors of the first group being coupled to one another to form a first Wheatstone bridge. A second group of piezoresistors is provided in the diaphragm proximate a second outer edge of the diaphragm, the piezoresistors of the second group being coupled to one another to form a second Wheatstone bridge. The first and second Wheatstone bridges exhibit mirror symmetry relative to one another. Output signals from each of the first and second Wheatstone bridges are processed at respective first and second differential amplifiers. The output signals from each of the first and second differential amplifiers are processed at a third differential amplifier to produce a pressure output signal with enhanced sensitivity and reduced impact from process variation.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to microelectromechanicalsystems (MEMS) pressure sensors. More specifically, the presentinvention relates to a MEMS pressure sensor having a multiple Wheatstonebridge configuration of sense elements for enhanced sensitivity andimproved robustness to process variations.

BACKGROUND OF THE INVENTION

Conventional piezoresistive pressure sensors are formed by a Wheatstonebridge that includes four piezoresistors. These four piezoresistors areplaced near the edge of a deformable membrane, i.e., a diaphragm, wherethe stress change is high under external pressure. Of the fourpiezoresistors, two may be symmetrically positioned parallel to theadjacent diaphragm edge, and the other two may be symmetricallypositioned normal, i.e., perpendicular, to the adjacent diaphragm edge.Under external pressure, the piezoresistors in parallel to the adjacentdiaphragm edge are subjected to opposite stresses relative to the twopiezoresistors that are normal to the adjacent diaphragm edge. Thisresults in increased resistance for the piezoresistors that are normaland decreased resistance for the piezoresistors that are parallel.Accordingly, the output voltage of the Wheatstone bridge changes withexternal applied pressure.

In general, there are two types of pressure sensor designs based onWheatstone bridges. One type of design places all four piezoresistors ofthe Wheatstone bridge proximate one edge of the diaphragm. The othertype of design places one piezoresistor of the Wheatstone bridge on eachof the four edges of the diaphragm. In either configuration, anelectronic circuit detects the resistance changes of the piezoresistivebridge and outputs an electrical signal representative of the externalapplied pressure.

Increasing the pressure sensitivity of a pressure sensor may provideimproved resolution and therefore yield improved device performance.Device sensitivity may be increased by increasing the size of adiaphragm. That is, a bigger diaphragm can provide higher deflectionunder a given applied external pressure and generate more change instress at the piezoresistor locations. More change in stress at thepiezoresistor locations produces a larger electrical output, thereforeincreased sensitivity. However, a larger diaphragm has the disadvantagesof more fragile die/wafer, larger die size/higher cost, and degradedlinearity performances.

Further, prior art piezoresistive pressure sensors can be very sensitiveto process variations, especially the alignment of piezoresistorlocations to the pressure cavity. That is, during fabrication, there canbe a misalignment between the backside cavity of a pressure sensor andthe piezoresistors formed in the diaphragm suspended across the cavity.Since the piezoresistors are placed symmetrically along the edge of thediaphragm, any cavity misalignment may translate to an asymmetry of thepiezoresistors which can result in an output offset of the Wheatstonebridge.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures in which like reference numerals refer toidentical or functionally similar elements throughout the separateviews, the figures are not necessarily drawn to scale, and whichtogether with the detailed description below are incorporated in andform part of the specification, serve to further illustrate variousembodiments and to explain various principles and advantages all inaccordance with the present invention.

FIG. 1 shows a side view of a pressure sensor in a simplified form;

FIG. 2 shows a top view of a pressure sensing element of the pressuresensor of FIG. 1 in accordance with an embodiment;

FIG. 3 shows a circuit diagram of the pressure sensor of FIG. 1; and

FIG. 4 shows a top view of a pressure sensing element of the pressuresensor of FIG. 1 in accordance with another embodiment.

DETAILED DESCRIPTION

In overview, the present disclosure concerns a pressure sensor withenhanced sensitivity and improved robustness to process variations. Moreparticularly, the pressure sensor includes a multiple Wheatstone bridgeconfiguration of piezoresistors that enable enhanced sensitivity andyield improvements in robustness to process variations. A pressuresensor design with reduced adverse impact from process variation canimprove device performance and reproducibility. Further, the pressuresensor design may enable the reduction of test insertions, and,therefore test time, over a variety of temperatures and pressures. Assuch, various inventive concepts and principles embodied in the pressuresensor may improve product quality while enabling cost savings.

The instant disclosure is provided to further explain in an enablingfashion the best modes, at the time of the application, of making andusing various embodiments in accordance with the present invention. Thedisclosure is further offered to enhance an understanding andappreciation for the inventive principles and advantages thereof, ratherthan to limit in any manner the invention. The invention is definedsolely by the appended claims including any amendments made during thependency of this application and all equivalents of those claims asissued.

It should be understood that the use of relational terms, if any, suchas first and second, top and bottom, and the like are used solely todistinguish one from another entity or action without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions.

Referring to FIG. 1, FIG. 1 shows a side view of a pressure sensor 20 ina simplified form. In general, pressure sensor 20 includes a pressuresensing element 22 and an application specific integrated circuit (ASIC)24 electrically connected to pressure sensing element 22 via a pluralityof bond wires 26 (one visible). Pressure sensing element 22 and ASIC 24may, in some configurations, be coupled to a common base 28. Pressuresensing element 22 generally includes a substrate 30 having a cavity 32.

A deformable membrane, referred to herein as a diaphragm 34, issuspended across cavity 32. In some embodiments, pressure sensingelement 22 and ASIC 24 may be encapsulated in a molding compound 36. Aport 38 may thus extend through molding compound 36 so as to exposediaphragm 34 to an external applied pressure, P, 40. Under externalapplied pressure 40, diaphragm 34 deforms. ASIC 24 detects theresistance changes of piezoresistors (discussed below) provided indiaphragm 34 and outputs an electrical signal representative of externalapplied pressure 40.

FIG. 2 shows a top view of pressure sensing element 22 of pressuresensor 20 (FIG. 1) in accordance with an embodiment. In particular,diaphragm 34 is shown with a plurality of sense elements, e.g.,piezoresistors, provided therein. Diaphragm 34 has a first outer edge 42and a second outer edge 44 disposed on opposing sides of a surface 46 ofdiaphragm 34. Diaphragm 34 further has a third outer edge 48 and afourth outer edge 50 disposed on opposing sides of surface 46 ofdiaphragm 34 and interposed between first and second outer edges 42, 44.Thus, surface 46 is circumscribed by first, second, third, and fourthouter edges 42, 44, 48, 50. A dashed line box represents the outerperimeter of cavity 32 underlying diaphragm 34.

In an embodiment, pressure sensing element 22 includes multipleWheatstone bridges circuits, each of which includes four piezoresistorsthat are placed together near one outer edge of diaphragm 34. In thisexample, a first group of piezoresistors is provided in diaphragm 34proximate first outer edge 42 of diaphragm 34. The piezoresistors of thefirst group are coupled to one another to form a first Wheatstone bridge52. A second group of piezoresistors is provided in diaphragm 34proximate second outer edge 44 of diaphragm 34. The piezoresistors ofthe second group are coupled to one another to form a second Wheatstonebridge 54. Similarly, a third group of piezoresistors is provided indiaphragm 34 proximate third outer edge 48 of diaphragm 34. Thepiezoresistors of the third group are coupled to one another to form athird Wheatstone bridge 56. A fourth group of piezoresistors is providedin diaphragm 34 proximate fourth outer edge 50 of diaphragm 34. Thepiezoresistors of the fourth group are coupled to one another to form afourth Wheatstone bridge 58.

Each of first, second, third, and fourth Wheatstone bridges 52, 54, 56,58 is approximately equidistant from its respective, first, second,third, and fourth outer edges 42, 44, 48, 50. In an ideal configuration,diaphragm 34 would be centered over cavity 32 (as shown in FIG. 2) sothat each of first, second, third, and fourth Wheatstone bridges 52, 54,56, 58 is subject to the same stress level on surface 46 of diaphragm asa function of the distance from a center 60 of cavity 32 in response toexternal applied pressure 40 (FIG. 1). However, process variation canresult can result in misalignment of diaphragm 34 to cavity 32, andtherefore misalignment of Wheatstone bridges 52, 54, 56, 58 to center 60of cavity 32. Nevertheless, the configuration of piezoresistors inWheatstone bridges 52, 54, 56, 68 largely cancels any output offset inthe event of a misalignment of diaphragm 34 to cavity 32, as will bediscussed in greater detail below.

With particular reference to the structure and arrangement of theWheatstone bridges, first Wheatstone bridge 52 includes a first leg 62having a first piezoresistor 64, labeled R₁, a second leg 66 having asecond piezoresistor 68, labeled R₂, a third leg 70 having a thirdpiezoresistor 72, labeled R₃, and a fourth leg 74 having a fourthpiezoresistor 76, labeled R₄. First and fourth piezoresistors 64, 76 arecoupled in series to form a first half of first Wheatstone bridge 52.Second and third piezoresistors 68, 72 are coupled in series to form asecond half of first Wheatstone bridge 52. The first half of firstWheatstone bridge 52 is coupled in parallel with the second half offirst Wheatstone bridge 52 such that a first node between first andsecond piezoresistors 64, 68 forms a first input terminal 78, labeledV_(P), and a second node between third and fourth piezoresistors 72, 76forms a second input terminal 80, labeled V_(N). In accordance with theWheatstone bridge configuration, a third node between first and fourthpiezoresistors 64, 76 forms a first positive output terminal 82, labeledS_(P1), and a fourth node between second and third piezoresistors 68, 72forms a first negative output terminal 84, labeled S_(N1). It can bereadily observed that first and third legs 62, 70 are orientedsubstantially parallel to first outer edge 42 of diaphragm 34.Additionally, second and fourth legs 66, 74 are oriented substantiallynormal, i.e., perpendicular, to first outer edge 42 of diaphragm 34.

Now with reference to second Wheatstone bridge 54, second Wheatstonebridge 54 includes a first leg 86 having a first piezoresistor 88,labeled R₇, a second leg 90 having a second piezoresistor 92, labeledR₈, a third leg 94 having a third piezoresistor 96, labeled R₅, and afourth leg 98 having a fourth piezoresistor 100, labeled R₆. First andfourth piezoresistors 88, 100 are coupled in series to form a first halfof second Wheatstone bridge 54. Second and third piezoresistors 92, 96are coupled in series to form a second half of second Wheatstone bridge54. The first half of second Wheatstone bridge 54 is coupled in parallelwith the second half of second Wheatstone bridge 54 such that a firstnode between first and second piezoresistors 88, 92 forms a first inputterminal 102, labeled V_(P), and a second node between third and fourthpiezoresistors 96, 100 forms a second input terminal 104, labeled V_(N).In accordance with the Wheatstone bridge configuration, a third nodebetween first and fourth piezoresistors 88, 100 forms a second positiveoutput terminal 106, labeled S_(P2), and a fourth node between secondand third piezoresistors 92, 96 forms a second negative output terminal108, labeled S_(N2). Again, it can be readily observed that first andthird legs 86, 94 are oriented substantially parallel to second outeredge 44 of diaphragm 34. Additionally, second and fourth legs 90, 98 areoriented substantially normal, i.e., perpendicular, to second outer edge44 of diaphragm 34.

Of particular interest herein, first Wheatstone bridge 52 and secondWheatstone bridge 54 exhibit mirror symmetry relative to one anotheralong a first line, or axis, of symmetry 110 that is parallel to surface46 of diaphragm 34. Mirror symmetry, also referred to as reflectionsymmetry, is symmetry with respect to reflection. By way of explanation,if diaphragm 34 was folded in half at line of symmetry 110, the twohalves of diaphragm 34 would be identical, i.e., the two halves are eachother's mirror images. As such, any feature of first Wheatstone bridge52 is the same distance away from first line of symmetry 110 as the samefeature of second Wheatstone bridge 54. Accordingly, first, second,third, and fourth legs 86, 90, 94, 98 of second Wheatstone bridge 54 aredisposed the same distance away from first line of symmetry 110, and arein the same relative positions, as first, second, third, and fourth legs62, 66, 70, 74 of first Wheatstone bridge 52. Likewise, first and secondinput terminals 102, 104, second positive output terminal 106, andsecond negative output terminal 108 are disposed the same distance awayfrom first line of symmetry 110, and are in the same relative positions,as first and second input terminals 78, 80, first positive outputterminal 82, and first negative output terminal 84.

Referring now to third Wheatstone bridge 56, third Wheatstone bridge 56includes a first leg 112 having a first piezoresistor 114, labeled R₉, asecond leg 116 having a second piezoresistor 118, labeled R₁₀, a thirdleg 120 having a third piezoresistor 122, labeled R₁₁, and a fourth leg124 having a fourth piezoresistor 126, labeled R₁₂. First and fourthpiezoresistors 114, 126 are coupled in series to form a first half ofthird Wheatstone bridge 56. Second and third piezoresistors 118, 122 arecoupled in series to form a second half of third Wheatstone bridge 56.The first half of third Wheatstone bridge 56 is coupled in parallel withthe second half of third Wheatstone bridge 56 such that a first nodebetween first and second piezoresistors 114, 118 forms a first inputterminal 128, labeled V_(P), and a second node between third and fourthpiezoresistors 122, 126 forms a second input terminal 130, labeledV_(N). In accordance with the Wheatstone bridge configuration, a thirdnode between first and fourth piezoresistors 114, 126 forms a thirdpositive output terminal 132, labeled S_(P3), and a fourth node betweensecond and third piezoresistors 118,122 forms a third negative outputterminal 134, labeled S_(N3). It can be readily observed that first andthird legs 112, 120 are oriented substantially parallel to third outeredge 48 (and hence perpendicular to first and second outer edges 42, 44)of diaphragm 34. Additionally, second and fourth legs 116, 124 areoriented substantially normal, i.e., perpendicular, to third outer edge48 (and hence parallel to first and second outer edges 42, 44) ofdiaphragm 34.

Fourth Wheatstone bridge 58 includes a first leg 136 having a firstpiezoresistor 138, labeled R₁₅, a second leg 140 having a secondpiezoresistor 142, labeled R₁₆, a third leg 144 having a thirdpiezoresistor 146, labeled R₁₃, and a fourth leg 148 having a fourthpiezoresistor 150, labeled R₁₄. First and fourth piezoresistors 138, 150are coupled in series to form a first half of fourth Wheatstone bridge58. Second and third piezoresistors 142, 146 are coupled in series toform a second half of fourth Wheatstone bridge 58. The first half offourth Wheatstone bridge 58 is coupled in parallel with the second halfof fourth Wheatstone bridge 58 such that a first node between first andsecond piezoresistors 138, 142 forms a first input terminal 152, labeledV_(P), and a second node between third and fourth piezoresistors 146,150 forms a second input terminal 154, labeled V_(N). In accordance withthe Wheatstone bridge configuration, a fourth node between first andfourth piezoresistors 138, 150 forms a fourth positive output terminal156, labeled S_(P4), and a fourth node between second and thirdpiezoresistors 142, 146 forms a fourth negative output terminal 158,labeled S_(N4). Again, it can be readily observed that first and thirdlegs 136, 144 are oriented substantially parallel to fourth outer edge50 (and hence perpendicular to first and second outer edges 42, 44) ofdiaphragm 34. Additionally, second and fourth legs 140, 148 are orientedsubstantially normal, i.e., perpendicular, to fourth outer edge 50 (andhence parallel to first and second outer edges 42, 44) of diaphragm 34.

Third Wheatstone bridge 56 and fourth Wheatstone bridge 58 exhibitmirror symmetry relative to one another along a second line, or axis, ofsymmetry 160. Like first line of symmetry 110, second line of symmetryis parallel to surface 46 of substrate 34. However, second line ofsymmetry 160 is orthogonal to first line of symmetry 110. Accordingly,first, second, third, and fourth legs 136, 140, 144, 148 of fourthWheatstone bridge 58 are disposed the same distance away from secondline of symmetry 160, and are in the same relative positions, as first,second, third, and fourth legs 112, 116, 120, 124 of third Wheatstonebridge 56. Likewise, first and second input terminals 152, 154, fourthpositive output terminal 156, and fourth negative output terminal 158 offourth Wheatstone bridge 58 are disposed the same distance away fromsecond line of symmetry 160, and are in the same relative positions, asfirst and second input terminals 128, 130, third positive outputterminal 132, and third negative output terminal 134 of third Wheatstonebridge 56.

Now referring to FIG. 3 concurrently with FIG. 2, FIG. 3 shows a circuitdiagram of pressure sensor 20 (FIG. 1) having the multiple Wheatstonebridge configuration shown in FIG. 2. In particular, FIG. 3 shows first,second, third, and fourth Wheatstone bridges 52, 54, 56, 58 of pressuresensing element 22 and their interconnection with multiple differentialamplifiers of ASIC 24.

When a voltage is input to first input terminals 78, 80 (V_(P), V_(N))of first Wheatstone bridge 52, the output voltage at first outputterminals 82, 84 (S_(P1), S_(N1)) changes in response to changes in thevalues of first, second, third, and fourth piezoresistors 64, 68, 72, 76(R₁, R₂, R₃, R₄). Similarly, when voltage is input to first inputterminals 102, 104 (V_(P), V_(N)) of second Wheatstone bridge 54, theoutput voltage at second output terminals 106, 108 (S_(P2), S_(N2))changes in response to changes in the values of first, second, third,and fourth piezoresistors 88, 92, 96, 100 (R₇, R₈, R₅, R₆). When voltageis input to first input terminals 128, 130 (V_(P), V_(N)) of thirdWheatstone bridge 56, the output voltage at third output terminals 132,134 (S_(P3), S_(N3)) changes in response to changes in the values offirst, second, third, and fourth piezoresistors 114, 118, 122, 126 (R₉,R₁₀, R₁₁, R₁₂). When voltage is input to first input terminals 152, 154(V_(P), V_(N)) of fourth Wheatstone bridge 58, the output voltage atfourth output terminals 156, 158 (S_(P4), S_(N4)) changes in response tochanges in the values of first, second, third, and fourth piezoresistors138, 142, 146, 150 (R₁₅, R₁₆, R₁₃, R₁₄). Since piezoresistors 64, 68,72, 76, 88, 92, 96, 100, 114, 118, 122, 126, 138, 142, 146, 150 areformed in diaphragm 34 of pressure sensing element 22, the nominalresistance of each of the piezoresistors will change when diaphragm 34deflects in response to external pressure 40 (FIG. 1).

In an embodiment, ASIC 24 includes a first differential amplifier 162,labeled A1, having a first positive terminal 164, a first negativeterminal 166, and a first output terminal 168. First positive terminal164 is coupled to first positive output terminal 82 of first Wheatstonebridge 52 and first negative terminal 166 is coupled to first negativeoutput terminal 84 of first Wheatstone bridge 52. First differentialamplifier 162 amplifies the difference between the two input voltages atfirst positive and negative terminals 164, 166 and suppresses anyvoltage common to first positive and negative terminals 164, 166. Thus,a first signal, referred to herein as a first voltage 170, V_(A1), isoutput from first output terminal 168 of first differential amplifier162 and is proportional to the difference between the two voltages asV_(A1)=A(S_(P1)−S_(N1)), where “A” represents the gain of firstdifferential amplifier 162.

ASIC 24 includes a second differential amplifier 172, labeled A2, havinga second positive terminal 174, a second negative terminal 176, and asecond output terminal 178. Second positive terminal 174 is coupled tosecond negative output terminal 108 of second Wheatstone bridge 54 andsecond negative terminal 176 is coupled to second positive outputterminal 106 of second Wheatstone bridge 54. Second differentialamplifier 172 amplifies the difference between the two input voltages atsecond positive and negative terminals 174, 176 and suppresses anyvoltage common to second positive and negative terminals 174, 176. Thus,a second signal, referred to herein as a second voltage 180, V_(A2), isoutput from second output terminal 178 and is proportional to thedifference between the two voltages as V_(A2)=A(S_(P2)−S_(N2)), where“A” represents the gain of second differential amplifier 172.

ASIC 24 includes a third differential amplifier 182, labeled A3, havinga third positive terminal 184, a third negative terminal 186, and athird output terminal 188. Third positive terminal 184 is coupled tothird positive output terminal 132 of third Wheatstone bridge 56 andthird negative terminal 186 is coupled to third negative output terminal134 of third Wheatstone bridge 56. Third differential amplifier 182amplifies the difference between the two input voltages at thirdpositive and negative terminals 184, 186 and suppresses any voltagecommon to third positive and negative terminals 184, 186. Thus, a thirdsignal, referred to herein as a third voltage 190, V_(A3), is outputfrom third output terminal 188 and is proportional to the differencebetween the two voltages as V_(A3)=A(S_(P3)−S_(N3)), where “A”represents the gain of third differential amplifier 182.

ASIC 24 includes a fourth differential amplifier 192, labeled A4, havinga fourth positive terminal 194, a fourth negative terminal 196, and afourth output terminal 198. Fourth positive terminal 194 is coupled tofourth negative output terminal 158 of fourth Wheatstone bridge 58 andfourth negative terminal 196 is coupled to fourth positive outputterminal 156 of fourth Wheatstone bridge 58. Fourth differentialamplifier 192 amplifies the difference between the two input voltages atfourth positive and negative terminals 194, 196 and suppresses anyvoltage common to fourth positive and negative terminals 194, 196. Thus,a fourth signal, referred to herein as a fourth voltage 200, V_(A4), isoutput from fourth output terminal 198 and is proportional to thedifference between the two voltages as V_(A4)=A(S_(P4)−S_(N4)), where“A” represents the gain of fourth differential amplifier 192.

ASIC 24 further includes a fifth differential amplifier 202, labeled B1,having a fifth positive terminal 204, a fifth negative terminal 206, anda fifth output terminal 208. Fifth positive terminal 204 is coupled tofirst output terminal 168 of first differential amplifier 162 and fifthnegative terminal 206 is coupled to second output terminal 178 of seconddifferential amplifier 172. Fifth differential amplifier 202 amplifiesthe difference between the two input voltages at fifth positive andnegative terminals 204, 206 and suppresses any voltage common to fifthpositive and negative terminals 204, 206. Thus, a fifth signal, referredto herein as a fifth voltage 210, V_(B1), is output from fifth outputterminal 208 and is proportional to an amplified difference between thetwo voltages as V_(B1)=A(V_(A1)−V_(A2)), where “A” represents the gainof fifth differential amplifier 202.

ASIC 24 further includes a sixth differential amplifier 212, labeled B2,having a sixth positive terminal 214, a sixth negative terminal 216, anda sixth output terminal 218. Sixth positive terminal 214 is coupled tothird output terminal 188 of third differential amplifier 182 and sixthnegative terminal 216 is coupled to fourth output terminal 198 of fourthdifferential amplifier 192. Sixth differential amplifier 212 amplifiesthe difference between the two input voltages at sixth positive andnegative terminals 214, 216 and suppresses any voltage common to sixthpositive and negative terminals 214, 206. Thus, a sixth signal, referredto herein as a sixth voltage 220, V_(B2), is output from sixth outputterminal 218 and is proportional to an amplified difference between thetwo voltages as V_(B2)=A(V_(A3)−V_(A4)), where “A” represents the gainof sixth differential amplifier 212.

First and second Wheatstone bridges 52, 54 will have a similar outputbut with opposite polarities under the same external applied pressure 40(FIG. 1) due to their mirror symmetry configuration. Likewise, third andfourth Wheatstone bridges 56, 58 will have a similar output but withopposite polarities under the same external applied pressure 40 due totheir mirror symmetry configuration. For simplicity when the gain ofeach of differential amplifiers 162 172, 182, 192, 202, 212 is set toone, then in an ideal situation, V_(A1)=−V_(A2)=V_(A3)=V_(A4). Theoutput voltage, V_(OUT), can be determined as follows:

V _(OUT) =V _(B1) −V _(B2)   (1)

V _(OUT) =V _(A1)−(−V _(A2))+V _(A3)−(−V _(A4))   (2)

Accordingly, it can be readily observed that the output of pressuresensor 20 (FIG. 1) having the four Wheatstone bridge configuration maybe four times greater than the single Wheatstone bridge configuration ofprior art pressure sensors. Additionally, due to the mirror symmetryconfiguration of first and second Wheatstone bridges 52, 54 and thirdand fourth Wheatstone bridges 56, 58 and their locations in diaphragm 34(FIG. 2), any output offset due to process variation may be canceled orsignificantly reduced. Of course, once the output voltage, V_(OUT), isproduced further electronic circuitry contained within ASIC 24 maysuitably condition and convert the analog output voltage to a digitalsignal in accordance with known methodologies.

FIG. 4 shows a top view of a pressure sensing element 222 of pressuresensor 20 (FIG. 1) in accordance with another embodiment. Pressuresensing element 222 includes all of the features of pressure sensingelement 22 (FIG. 2). That is, pressure sensing element 222 includesdiaphragm 34 suspended across cavity 32. Pressure sensing element 222additionally includes a first Wheatstone bridge 224 of piezoresistivesense elements and a second Wheatstone bridge 226 of piezoresistivesense elements, in which the features of first and second Wheatstonebridges 224, 226 are arranged in mirror symmetry relative to a line ofsymmetry 228. Additionally, pressure sensing element 222 includes athird Wheatstone bridge 230 of piezoresistive sense elements and afourth Wheatstone bridge 232 of piezoresistive sense elements, in whichthe features of third and fourth Wheatstone bridges 230, 232 arearranged in mirror symmetry relative to another line of symmetry 234.

The circuit diagram of FIG. 3 still applies to the configuration ofWheatstone bridges of pressure sensing element 222. Accordingly, thedifferential amplifiers described in connection with FIG. 3 could stillbe implemented to suitably process and combine signals to get thevoltage output of the multiple Wheatstone bridge configuration ofpressure sensing element 222. However, the configuration of Wheatstonebridges 224, 226, 230, 232 of pressure sensing element 222 enables thesimplification of signal routing of the inputs, i.e., V_(P) and V_(N),and signal outputs, S_(P1), S_(N1), S_(P2), S_(N2), S_(P3), S_(N3),S_(P4), S_(N4).

Thus, embodiments disclosed herein entail a pressure sensor withenhanced sensitivity and improved robustness to process variations. Anembodiment of a pressure sensor comprises a substrate having a cavityand a diaphragm suspended across the cavity, the diaphragm having firstand second outer edges disposed on opposing sides of a surface of thediaphragm. A first group of sense elements is provided in the diaphragmproximate the first outer edge, the sense elements of the first groupbeing coupled to one another to form a first Wheatstone bridge, and asecond group of sense elements is provided in the diaphragm proximatethe second outer edge, the sense elements of the second group beingcoupled to one another to form a second Wheatstone bridge.

Another embodiment of a pressure sensor comprises a substrate having acavity and a diaphragm suspended across the cavity, the diaphragm havingfirst and second outer edges disposed on opposing sides of a surface ofthe diaphragm. A first group of piezoresistors is provided in thediaphragm proximate the first outer edge, the piezoresistors of thefirst group being coupled to one another to form a first Wheatstonebridge, and a second group of piezoresistors is provided in thediaphragm proximate the second outer edge, the piezoresistors of thesecond group being coupled to one another to form a second Wheatstonebridge. The first Wheatstone bridge and the second Wheatstone bridgeexhibit mirror symmetry relative to one another along a line ofsymmetry, and each of the first and second Wheatstone bridges isconfigured to produce an output signal in response to movement of thediaphragm responsive to an external pressure.

Yet another embodiment of a pressure sensor comprises a substrate havinga cavity and a diaphragm suspended across the cavity, the diaphragmhaving first and second outer edges disposed on opposing sides of asurface of the diaphragm, and the diaphragm having third and fourthsecond outer edges disposed on opposing sides of the surface of thediaphragm and interposed between the first and second outer edges sothat the surface of the diaphragm is circumscribed by the first, second,third, and fourth outer edges. A first group of piezoresistors isprovided in the diaphragm proximate the first outer edge, thepiezoresistors of the first group being coupled to one another to form afirst Wheatstone bridge, and a second group of piezoresistors isprovided in the diaphragm proximate the second outer edge, thepiezoresistors of the second group being coupled to one another to forma second Wheatstone bridge. A third group of piezoresistors is providedin the diaphragm proximate the third outer edge, the piezoresistors ofthe third group being coupled to one another to form a third Wheatstonebridge, and a fourth group of piezoresistors is provided in thediaphragm proximate the fourth outer edge, the piezoresistors of thefourth group being coupled to one another to form a fourth Wheatstonebridge. Each of the first, second, third, and fourth Wheatstone bridgesis configured to produce an output signal in response to movement of thediaphragm responsive to an external pressure.

Accordingly, a pressure sensor that includes a multiple Wheatstonebridge configuration of piezoresistors enables enhanced sensitivity andyield improvements in robustness to process variations. The pressuresensor design with reduced adverse impact from process variation canimprove device performance and reproducibility. Further, the pressuresensor design may enable the reduction of test insertions, and,therefore test time, over a variety of temperatures and pressure. Assuch, various inventive concepts and principles embodied in the pressuresensor may improve product quality while enabling cost savings.

This disclosure is intended to explain how to fashion and use variousembodiments in accordance with the invention rather than to limit thetrue, intended, and fair scope and spirit thereof. The foregoingdescription is not intended to be exhaustive or to limit the inventionto the precise form disclosed. Modifications or variations are possiblein light of the above teachings. The embodiment(s) was chosen anddescribed to provide the best illustration of the principles of theinvention and its practical application, and to enable one of ordinaryskill in the art to utilize the invention in various embodiments andwith various modifications as are suited to the particular usecontemplated. All such modifications and variations are within the scopeof the invention as determined by the appended claims, as may be amendedduring the pendency of this application for patent, and all equivalentsthereof, when interpreted in accordance with the breadth to which theyare fairly, legally, and equitably entitled.

What is claimed is:
 1. A pressure sensor comprising: a substrate havinga cavity; a diaphragm suspended across said cavity, said diaphragmhaving first and second outer edges disposed on opposing sides of asurface of said diaphragm; a first group of sense elements provided insaid diaphragm proximate said first outer edge, said sense elements ofsaid first group being coupled to one another to form a first Wheatstonebridge; and a second group of sense elements provided in said diaphragmproximate said second outer edge, said sense elements of said secondgroup being coupled to one another to form a second Wheatstone bridge.2. The pressure sensor of claim 1 wherein said sense elements of saidfirst and second groups comprise piezoresistors, and each of said firstand second Wheatstone bridges is configured to produce an output signalin response to movement of said diaphragm responsive to an externalpressure.
 3. The pressure sensor of claim 1 wherein said firstWheatstone bridge and said second Wheatstone bridge exhibit mirrorsymmetry relative to one another along a line of symmetry.
 4. Thepressure sensor of claim 1 wherein each of said first and secondWheatstone bridge comprises: a first sensor leg having a first one ofsaid sense elements; a second leg having a second one of said senseelements; a third leg having a third one of said sense element; and afourth leg having a fourth one of said sense elements, said first andfourth sense elements being coupled in series to form a first half ofsaid each of said first and second Wheatstone bridges, said second andthird sense elements being coupled in series to form a second half ofsaid each of said first and second Wheatstone bridges, said first halfbeing coupled in parallel with said second half such that a first nodebetween said first and second sense elements forms a first inputterminal and a second node between said third and fourth sense elementsforms a second input terminal, and wherein said first and third legs aresubstantially parallel to said first and second outer edges, and saidsecond and fourth legs are substantially perpendicular to said first andsecond outer edges.
 5. The pressure sensor of claim 1 wherein: saiddiaphragm further has third and fourth second outer edges disposed onopposing sides of said surface of said diaphragm and interposed betweensaid first and second outer edges so that said surface of said diaphragmis circumscribed by said first, second, third, and fourth outer edges;and said pressure sensor further comprises: a third group of senseelements provided in said diaphragm proximate said third outer edge,said sense elements of said third group being coupled to one another toform a third Wheatstone bridge; and a fourth group of sense elementsprovided in said diaphragm proximate said fourth outer edge, said senseelements of said fourth group being coupled to one another to form afourth Wheatstone bridge.
 6. The pressure sensor of claim 5 wherein:said first Wheatstone bridge and said second Wheatstone bridge exhibitmirror symmetry relative to one another along a first line of symmetry;and said third Wheatstone bridge and said fourth Wheatstone bridgeexhibit mirror symmetry relative to one another along a second line ofsymmetry, said second line of symmetry being orthogonal to said firstline of symmetry.
 7. The pressure sensor of claim 6 wherein said firstand second lines of symmetry are substantially parallel to said surfaceof said diaphragm.
 8. The pressure sensor of claim 1 wherein each ofsaid first and second Wheatstone bridges has a positive output node anda negative output node, and said pressure sensor further comprises: afirst differential amplifier having a first positive terminal, a firstnegative terminal, and a first output terminal, said first positiveterminal being coupled to said positive output node of said firstWheatstone bridge, said first negative terminal being coupled to saidnegative output node of said first Wheatstone bridge, said first outputterminal providing a first signal responsive to an external pressuredetected by said first Wheatstone bridge; and a second differentialamplifier having a second positive terminal, a second negative terminal,and a second output terminal, said second positive terminal beingcoupled to said negative output node of said second Wheatstone bridge,said second negative terminal being coupled to said positive output nodeof said second Wheatstone bridge, and said second output terminalproviding a second signal responsive to said external pressure detectedby said second Wheatstone bridge.
 9. The pressure sensor of claim 8wherein said second signal is opposite in polarity from said firstsignal in response to said external pressure.
 10. The pressure sensor ofclaim 8 wherein: said diaphragm further has third and fourth secondouter edges disposed on opposing sides of said surface of said diaphragmand interposed between said first and second outer edges so that saidsurface of said diaphragm is circumscribed by said first, second, third,and fourth outer edges; and said pressure sensor further comprises: athird group of sense elements provided in said diaphragm proximate saidthird outer edge, said sense elements of said third group being coupledto one another to form a third Wheatstone bridge; a fourth group ofsense elements provided in said diaphragm proximate said fourth outeredge, said sense elements of said fourth group being coupled to oneanother to form a fourth Wheatstone bridge, each of said third andfourth Wheatstone bridges having said positive output node and saidnegative output node; a third differential amplifier having a thirdpositive terminal, a third negative terminal, and a third outputterminal, said third positive terminal being coupled to said positiveoutput node of said third Wheatstone bridge, said third negativeterminal being coupled to said negative output node of said thirdWheatstone bridge, said third output terminal providing a third signalresponsive to said external pressure detected by said third Wheatstonebridge; and a fourth differential amplifier having a fourth positiveterminal, a fourth negative terminal, and a fourth output terminal, saidfourth positive terminal being coupled to said negative output node ofsaid fourth Wheatstone bridge, said second negative terminal beingcoupled to said positive output node of said fourth Wheatstone bridge,and said fourth output terminal providing a fourth signal responsive tosaid external pressure detected by said fourth Wheatstone bridge. 11.The pressure sensor of claim 10 further comprising: a fifth differentialamplifier having a fifth positive terminal, a fifth negative terminal,and a fifth output terminal, said fifth positive terminal being coupledto said first output terminal of said first differential amplifier, saidfifth negative terminal being coupled to said second output terminal ofsaid second differential amplifier, and said fifth output terminalproviding a fifth signal as an amplified difference between said firstand second signals; and a sixth differential amplifier having a sixthpositive terminal, a sixth negative terminal, and a sixth outputterminal, said sixth positive terminal being coupled to said thirdoutput terminal of said third differential amplifier, said sixthnegative terminal being coupled to said fourth output terminal of saidfourth differential amplifier, and said sixth output terminal providinga sixth signal as an amplified difference between said third and fourthsignals.
 12. The pressure sensor of claim 11 wherein said sixth signalis opposite in polarity from said fifth signal, and a signal output fromsaid pressure sensor is an amplified difference between said fifth andsixth signals.
 13. A pressure sensor comprising: a substrate having acavity; a diaphragm suspended across said cavity, said diaphragm havingfirst and second outer edges disposed on opposing sides of a surface ofsaid diaphragm; a first group of piezoresistors provided in saiddiaphragm proximate said first outer edge, said piezoresistors of saidfirst group being coupled to one another to form a first Wheatstonebridge; and a second group of piezoresistors provided in said diaphragmproximate said second outer edge, said piezoresistors of said secondgroup being coupled to one another to form a second Wheatstone bridge,said first Wheatstone bridge and said second Wheatstone bridge exhibitmirror symmetry relative to one another along a line of symmetry, andeach of said first and second Wheatstone bridges is configured toproduce an output signal in response to movement of said diaphragmresponsive to an external pressure.
 14. The pressure sensor of claim 13wherein each of said first and second Wheatstone bridge comprises: afirst sensor leg having a first one of said piezoresistors; a second leghaving a second one of said piezoresistors; a third leg having a thirdone of said piezoresistors; and a fourth leg having a fourth one of saidpiezoresistors, said first and fourth piezoresistors being coupled inseries to form a first half of said each of said first and secondWheatstone bridges, said second and third piezoresistors being coupledin series to form a second half of said each of said first and secondWheatstone bridges, said first half being coupled in parallel with saidsecond half such that a first node between said first and secondpiezoresistors forms a first input terminal and a second node betweensaid third and fourth piezoresistors forms a second input terminal, andwherein said first and third legs are substantially parallel to saidfirst and second outer edges, and said second and fourth legs aresubstantially perpendicular to said first and second outer edges. 15.The pressure sensor of claim 13 wherein each of said first and secondWheatstone bridges has a positive output node and a negative outputnode, and said pressure sensor further comprises: a first differentialamplifier having a first positive terminal, a first negative terminal,and a first output terminal, said first positive terminal being coupledto said positive output node of said first Wheatstone bridge, said firstnegative terminal being coupled to said negative output node of saidfirst Wheatstone bridge, said first output terminal providing a firstsignal responsive to an external pressure detected by said firstWheatstone bridge; and a second differential amplifier having a secondpositive terminal, a second negative terminal, and a second outputterminal, said second positive terminal being coupled to said negativeoutput node of said second Wheatstone bridge, said second negativeterminal being coupled to said positive output node of said secondWheatstone bridge, and said second output terminal providing a secondsignal responsive to said external pressure detected by said secondWheatstone bridge.
 16. The pressure sensor of claim 15 wherein saidsecond signal is opposite in polarity from said first signal in responseto said external pressure.
 17. A pressure sensor comprising: a substratehaving a cavity; a diaphragm suspended across said cavity, saiddiaphragm having first and second outer edges disposed on opposing sidesof a surface of said diaphragm, and said diaphragm having third andfourth second outer edges disposed on opposing sides of said surface ofsaid diaphragm and interposed between said first and second outer edgesso that said surface of said diaphragm is circumscribed by said first,second, third, and fourth outer edges; a first group of piezoresistorsprovided in said diaphragm proximate said first outer edge, saidpiezoresistors of said first group being coupled to one another to forma first Wheatstone bridge; a second group of piezoresistors provided insaid diaphragm proximate said second outer edge, said piezoresistors ofsaid second group being coupled to one another to form a secondWheatstone bridge; a third group of piezoresistors provided in saiddiaphragm proximate said third outer edge, said piezoresistors of saidthird group being coupled to one another to form a third Wheatstonebridge; and a fourth group of piezoresistors provided in said diaphragmproximate said fourth outer edge, said piezoresistors of said fourthgroup being coupled to one another to form a fourth Wheatstone bridge,and each of said first, second, third, and fourth Wheatstone bridges isconfigured to produce an output signal in response to movement of saiddiaphragm responsive to an external pressure.
 18. The pressure sensor ofclaim 17 wherein: said first Wheatstone bridge and said secondWheatstone bridge exhibit mirror symmetry relative to one another alonga first line of symmetry; and said third Wheatstone bridge and saidfourth Wheatstone bridge exhibit mirror symmetry relative to one anotheralong a second line of symmetry, said second line of symmetry beingorthogonal to said first line of symmetry.
 19. The pressure sensor ofclaim 17 wherein each of said first, second, third, and fourthWheatstone bridges has a positive output node and a negative outputnode, and said pressure sensor further comprises: a first differentialamplifier having a first positive terminal, a first negative terminal,and a first output terminal, said first positive terminal being coupledto said positive output node of said first Wheatstone bridge, said firstnegative terminal being coupled to said negative output node of saidfirst Wheatstone bridge, said first output terminal providing a firstsignal responsive to an external pressure detected by said firstWheatstone bridge; and a second differential amplifier having a secondpositive terminal, a second negative terminal, and a second outputterminal, said second positive terminal being coupled to said negativeoutput node of said second Wheatstone bridge, said second negativeterminal being coupled to said positive output node of said secondWheatstone bridge, and said second output terminal providing a secondsignal responsive to said external pressure detected by said secondWheatstone bridge; a third differential amplifier having a thirdpositive terminal, a third negative terminal, and a third outputterminal, said third positive terminal being coupled to said positiveoutput node of said third Wheatstone bridge, said third negativeterminal being coupled to said negative output node of said thirdWheatstone bridge, said third output terminal providing a third signalresponsive to said external pressure detected by said third Wheatstonebridge; and a fourth differential amplifier having a fourth positiveterminal, a fourth negative terminal, and a fourth output terminal, saidfourth positive terminal being coupled to said negative output node ofsaid fourth Wheatstone bridge, said second negative terminal beingcoupled to said positive output node of said fourth Wheatstone bridge,and said fourth output terminal providing a fourth signal responsive tosaid external pressure detected by said fourth Wheatstone bridge. 20.The pressure sensor of claim 19 further comprising: a fifth differentialamplifier having a fifth positive terminal, a fifth negative terminal,and a fifth output terminal, said fifth positive terminal being coupledto said first output terminal of said first differential amplifier, saidfifth negative terminal being coupled to said second output terminal ofsaid second differential amplifier, and said fifth output terminalproviding a fifth signal as an amplified difference between said firstand second signals; and a sixth differential amplifier having a sixthpositive terminal, a sixth negative terminal, and a sixth outputterminal, said sixth positive terminal being coupled to said thirdoutput terminal of said third differential amplifier, said sixthnegative terminal being coupled to said fourth output terminal of saidfourth differential amplifier, and said sixth output terminal providinga sixth signal as an amplified difference between said third and fourthsignals, wherein said sixth signal is opposite in polarity from saidfifth signal, and a signal output from said pressure sensor is anamplified difference between said fifth and sixth signals.